According to IoT technology development, wireless tags are expected to be used in various scenes. It is already known that a system of detecting a position of a wireless tag by a wireless receiver exists. In this type of system, the orientation of a wireless tag needs to be detected in order to estimate the position of the wireless tag by a single wireless receiver.
There are methods of detecting the orientation of an object that emits radio waves using an antenna array. In these methods, the direction of arrival of radio waves is estimated based on the phase difference of received signals between antenna elements. The Multiple Signal Classification (MUSIC) method is a representative method of these technique.
With the methods using antenna array, in multipath environment, i.e., when multiple waves having coherence arrive, it is difficult to separate the multiple waves. The position detection of wireless tags is mainly performed inside buildings. Multipath is inevitable in indoor environments.
With antenna arrays of particular shapes, the multiple waves stemming from multipath can be separated by using spatial smoothing. However, the shapes of antenna are limited, and the greater number of antennas should be used. In addition, resolution will be degraded. Accordingly, the spatial smoothing is not suitable for the case where the number of antennas is less.
On the other hand, there is a different approach related to radar. It is already known that in the case where signals having different frequencies are transmitted and received, an error in angle of arrival (AoA) detected from a pulse having the largest received power is the statistically minimum. This method can be applied to detection of an AoA of a wireless tag in a system which frequency hopping is adopted, for example, Bluetooth (registered trademark).
However, in general, the frequencies of signals transmitted from a wireless tag change at long-term intervals, differently from radar. In addition, in the case where a person carries a wireless tag, the orientation of the wireless tag may change over time. If the orientation of the wireless tag changes, the conditions such as an antenna gain and polarization, etc., also change from the conditions under the previous frequency. In the case where a person carries a wireless tag, power may change due to factors other than the change in multipath interference state. If power changes due to factors other than the change in multipath interference state, an AoA may not be accurately selected based on the power.